1. Field of the Invention
The present invention is generally directed to improving amplifier efficiency through controlling amplifier operating angle. More specifically, the present invention is directed to a circuit and method for continually adjusting amplifier operating angle from Class A through Class B which can prevent driving the output into cutoff, thereby avoiding associated gain nonlinearities and improving output stage bandwidth while providing higher efficiencies corresponding to reduced operating angles.
2. Background
The basic concept of push-pull amplification can be described as an amplifier in which there are two similar signal branch circuits operating in phase opposition whose outputs are combined in a difference or summing circuit to produce an increased power output. Amplifier efficiency varies with the amplifier operating angle which can be adjusted by setting the amplifier bias point. Amplifier operating voltages are selected in the absence of applied signals to set a quiescent (Q) operating point which determines the amount of output signal swing that can occur without clipping the signal.
The Q point in a Class A amplifier is chosen such that the output device is always conducting (e.g. collector current in a bipolar transistor or drain current in a field-effect transistor) and the output signal can swing symmetrically over the full 360 degrees of a driving sine wave. The output stage bias current is greater than the maximum output current so that the output devices always conduct current. The Q point is biased in the middle of the DC load line which permits the full symmetrical fluctuation of the driving sine wave along the AC load line. This biasing method provides an amplifier with a potentially distortionless operating point which reproduces the entire input signal. However, the Class A amplifier is very inefficient due to the quiescent power dissipated in maintaining the Q point. Therefore, operating angles of less than 360 degrees are desired because they produce progressively higher efficiency as the operating angle is reduced.
The Q point in Class B amplifiers is chosen at cutoff such that current flows through the output device over half or 180 degrees of the driving sine wave. Class B amplifiers are more efficient than Class A amplifiers because the ratio of average DC current to load current is improved by the absence of quiescent bias current. However, in a Class B push-pull configuration the output stages in the amplifier conduct alternately with one device conducting during positive swings of the driving sine wave and the other device conducting during negative swings. This means the drive signal magnitude would need to be doubled to yield the same output as a similarly configured Class A amplifier. Furthermore, the Class B amplifier may result in crossover distortion as conduction passes from one device to the other, as well as other distortions and gain errors. For example, if the operating angle becomes less than 180 degrees a portion of the input waveform is not reproduced, resulting in distortion and gain error. Similarly, if the operating angle becomes more than 180 degrees the gain is approximately double for the portion of the input waveform where both devices conduct, again resulting in distortion and gain error. These distortion and gain errors are due to difficulties in maintaining the correct overlap point resulting from variations in process, temperature and supply voltage.
Class AB amplifiers are variations of Class B amplifiers and are biased in a manner that reduces much of the crossover distortion and associated nonlinearity found in Class B amplifiers. The Q point in Class AB amplifiers is biased close to the edge of cutoff resulting in an output for any slight input signal variation. In this case, distortionless operation requires that the total gain of the two sides when both are conducting be exactly equal to the gain of one side conducting alone. Each output device handles only a little more than half of the output waveform, and thus, like the Class B amplifier the Class AB amplifier would require the drive signals to be approximately doubled to yield the same output as a similarly configured Class A amplifier. However, Class AB amplifiers differ from Class B amplifiers in that maintaining the Q point requires a small bias current to flow even when there is no input signal. This current increases the power consumption of the Class AB amplifier, but it is nowhere near the power consumed in a Class A amplifier. The operating region for Class AB amplifiers is between 360 and 180 degrees, which provides an amplifier efficiency better than Class A but not as good as Class B.
Therefore, to yield the same output as the Class A amplifier, both the Class B and AB amplifiers would require an input signal approximately twice as large. Additionally, the output stages for the Class B and AB amplifiers may not have sufficient bandwidth to accurately trace the abrupt on/off transitions in the drive waveforms, resulting in output distortion. It is therefore clear that the disadvantages of presently available amplifiers include very few low-distortion operating points which are only suitable when hit with high accuracy, gain differences which vary from 2 to 1 (Class A to Class B) with operating points, and the requirement for increasing bandwidth in the output stages of amplifiers as efficiency proceeds beyond that of Class A amplifiers.
Accordingly, there exists a need for a method of operating push-pull amplifier output stages in the Class AB to Class B operating region (i.e. a conduction angle of less than 360 degrees down to 180 degrees) for signal inputs without driving the output devices abruptly into cutoff and thereby avoiding gain nonlinearities in the almost-cutoff transfer region. Additionally, the need exists in such amplifiers for the output stages to accurately trace input drive signals as the high frequency content of these signals is adjusted, thereby reducing distortion resulting from a failure of the output stage to trace the input waveform correctly.